The present invention relates generally to an improved method and system for controlling an apparatus or method employing the Czochralski process for growing crystals. More particularly, the present invention relates to an open-loop method and system for automatically controlling the semiconductor single crystal growth process. Even more particularly, the present invention relates to such an open-loop method and system which can be used for automatically controlling the growth of the endcone of a silicon single crystal and maintaining the zero dislocation state of the crystal.
Single crystal silicon, which is the starting material of most processes for the fabrication of semiconductor electronic components, is commonly prepared by the Czochralski process. In this process, polycrystalline silicon is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon and a single crystal is grown by slow extraction. After formation of a neck is complete, the diameter of the crystal is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached to form a taper, or crown, portion of the crystal. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter must be reduced gradually to form an endcone. When the diameter becomes small enough, the crystal is then separated from the melt.
The process by which the crystal is separated from the silicon melt can adversely affect the quality of the crystal under certain conditions. If the diameter of the crystal is not reduced sufficiently when separation from the melt occurs or if the diameter is reduced too rapidly or irregularly, the crystal will experience thermal shock. Such thermal shock can cause slip dislocations in the endcone which can propagate into the main body of the crystal.
Further, because the endcone of the crystal is typically discarded, it also is desirable to minimize the axial length of the endcone grown on the crystal. The endcone length, however, still must be sufficient to minimize thermal shock to the crystal when separation from the silicon melt occurs. The endcone growth process, therefore, must be carefully controlled to satisfy the often opposing goals of minimizing endcone waste, avoiding the creation of dislocations in the crystal upon separation from the melt, and maintaining an acceptable thermal history for the crystal.
The processes conventionally used to control the crystal growth process depend upon precise control of the process by a skilled crystal puller operator or sophisticated closed-loop control schemes, or both. For example, Maeda et al., U.S. Pat. No. 5,223,078, which is incorporated herein by reference, describes a closed-loop method for controlling the growth of the conical portion of the crystal adjacent the seed crystal (the taper) and requires the active measurement and adjustment of process variables during growth of the taper. In the Maeda method, the melt temperature and diameter of the taper of the crystal being grown are measured. The change rate of the diameter is calculated and this change rate together with the measured temperature are compared to preset target temperature and change rate values. The target temperature is then redetermined based on existing data from a target temperature data file and a target diameter change rate data file. The amount of electricity supplied to the heater is then controlled, preferably by PID action of a controller, to obtain the corrected target temperature.
Katsuoka et al., U.S. Pat. No. 4,973,377, which is incorporated herein by reference, describes a closed-loop method for controlling the diameter of the taper by controlling the melt temperature and the rotational speed of the crucible.
Watanabe et al., U.S. Pat. No. 4,876,438, which is incorporated herein by reference, describes a device for controlling the diameter of a crystal by controlling the power supplied to the heater and the pull rate. The device operates on a closed-loop feedback process wherein two process variables related to crystal diameter are measured during the growth of the crystal and appropriate control action is taken to maintain the desired diameter.
Araki, U.S. Pat. No. 5,288,363, which is incorporated herein by reference, describes a closed-loop method for controlling taper growth. In the Araki method, the deviation of the crystal diameter from a desired crystal diameter is monitored. The pull rate is adjusted to minimize the deviation. In addition, a correction value for the amount of power supplied to the melt heater is calculated based on fuzzy interference. The heater power then is adjusted in accordance with the correction value.
These approaches, however, are not entirely satisfactory. First, they can require expensive and complex process control equipment and techniques as well as significant maintenance. Second, they often are additionally dependent upon some degree of operator control for precise operation. Third, they often decrease the throughput of the crystals grown according to the process. Fourth, care must be taken where process variables are measured as a part of the closed-loop system to avoid contamination of the crystal or silicon melt by the measuring equipment. Fifth, errors in control can cause incorrect adjustments of the crystal pull rate and power rate which, in turn, can adversely affect the success of the crystal growth.
Accordingly, there is a need for a process for controlling the growth of a silicon single crystal that minimizes and simplifies process control equipment and operation requirements, that minimizes dependency on operator control, that minimizes endcone waste, that improves the uniformity of the thermal history of the crystal, and/or that improves the process yield without compromising on diameter control.